This is one of the best galleries for Hispanic art anywhere in Texas. They always have very interesting exhibits so make sure to go to their website or check the local Austin Chronicle to see what's happening when.
I've been to several exhibits there but the Mexican Posters that was traveling around the country a couple of years ago was by far the best I've ever been to. They also have a good selection of prints for sale and very good art book selection.
To find parking in this hard part of town, just head out to any side street off Congress and you'll likely find something easily.